Bioremediation and Bioeconomy

Bioremediation and Bioeconomy

Bioremediation and Bioeconomy

Edited by M.N.V. Prasad Department of Plant Sciences University of Hyderabad, Telangana, India

AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK 225 Wyman Street, Waltham, MA 02451, USA Copyright © 2016 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library For information on all Elsevier publications visit our website at http://store.elsevier.com/ ISBN: 978-0-12-802830-8

CHAPTER

BIOPROCESSES FOR WASTE AND WASTEWATER REMEDIATION FOR SUSTAINABLE ENERGY

21

G. Mohanakrishna, S. Srikanth, D. Pant VITO—Flemish Institute for Technological Research, Mol, Belgium

1 INTRODUCTION TO ENERGY THROUGH BIOREMEDIATION Bioremediation is a natural process that decomposes or detoxifies or removes pollutants by the action of biological catalyst, that occurs at different levels of the ecosystem. Bioremediation encompasses water, soil, and air environments. Pollution rates in these areas are increasing rapidly with the industrialization and evolution of human needs. In the early stages of microbiology development, various biological processes for treating pollutants were understood. As the importance of remediation or treatment is intensifying, different processes have been developed for the treatment, and it has also been observed that biological treatment processes are superior to the other physicochemical processes. Bioremediation disintegrates pollutants and debris, which are stacking up in the environment and causing deterioration. The benefits of bioremediation also extend from the environment to health, life, and the world economy. Since bioremediation proceeds with biological components, limited energy input and mild operating conditions are sufficient for the mineralization or detoxification of pollutants. Traditional wastewater treatment processes, such as activated sludge process (ASP), treat soluble organic materials and suspended solids present in low-strength wastewater efficiently, but the process demands a high amount of energy (Abbassi et al., 2000; Judd, 2010). Later, anaerobic digestion (AD) was discovered as an efficient treatment process that also generates methane as the value-added energy product. Treating a wide variety of wastewaters and solid waste are added advantages of AD. The diverse microbial metabolisms present in AD led to the process modification for generation of energy and other chemical products such as hydrogen, volatile organic products, alcohols, etc. This aspect became very encouraging in the search for nontraditional and renewable energy (Lettinga et al., 1980; Nishio and Nakashimada, 2007; Mohan et al., 2008a). Both ASP and AD are prokaryotic processes, demonstrated by bacteria. Besides bacteria, eukaryotic microorganisms such as algae and fungi are also involved in waste remediation. Photoautotrophic and heterotrophic metabolisms of the microalgae help in the treatment process, which is called phycoremediation. Mycoremediation is the process that proceeds by fungi, which are one of the best-known catalysts to treat complex organic materials (Pant and Adholeya, 2009, 2010). The global research fraternity is greatly focused on bioenergy generation from waste though fermentative process. Use of these processes has been increasing exponentially in the last decade, underscoring the potential of waste treatment for bioenergy generation (Figure 1).

Bioremediation and Bioeconomy. http://dx.doi.org/10.1016/B978-0-12-802830-8.00021-6 Copyright © 2016 Elsevier Inc. All rights reserved.

537

538

CHAPTER 21 BIOPROCESSES FOR WASTE AND WASTEWATER REMEDIATION

250

No. of research articles

200

150

100

0

1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

50

Publication year

FIGURE 1 Scopus research results number of articles published from 1980 to till early 2015 on the energy production from waste fermentation (keywords: energy AND fermentation AND waste).

Any kind of energy generated from waste is due to the oxidation of organic pollutants present in waste, which means that the energy generation efficiency of any process is always proportional to the waste/contaminant removal. Even though, the side reactions are possible with all the biological processes, they should be eliminated to transfer the energy generated from the oxidation of waste to the desired product (Elmekawy et al., 2013a). This is also the basic principle of sustainable development, where both treatment/remediation and energy generation will be merged and materialized. This chapter presents comprehensive information about various biological treatment or remediation processes that also generate energy in different forms.

2 VERSATILE COMPONENTS OF BIOENERGY GENERATION THROUGH REMEDIATION 2.1 C-SOURCES Biological energy generation is mostly possible during carbon metabolism by microorganisms. The organic matter that is present in the wastewater, solid waste, and agricultural-based biomass oxidized to simpler organic molecules, generates the energy (catabolic process) (Mohanakrishna and Mohan, 2013; Niessen et al., 2004; Madsen, 2011). Carbon dioxide (CO2) reduction and fixing to biomass or biomolecules also leads to energy equivalents such as saccharides, lipids, and chemicals (anabolism). Algae that grow in wastewaters and produce lipids and carbohydrate biomass exhibit mixotrophic metabolism (Mohan et al., 2011a; Gentili, 2014). Irrespective of biocatalyst, all the mechanisms are linked

2 VERSATILE COMPONENTS OF BIOENERGY GENERATION

539

to the organic/carbon pollutants emphasizing their importance. Two different types of substrates are available, solid and liquid wastes, for bioenergy-generating processes, that are mainly differ based on total solids, total carbon content, and moisture/water content (Pant et al., 2012; Mohan et al., 2011b). Diverse energy-producing trajectories from the biological treatment of waste and wastewater and their respective products are depicted in Figure 2 and Table 1.

2.1.1 Solid waste Solid waste contains low water content and high solids/organic matter that is not amenable for treatment. Based on solids concentration, it needs dilution with water or domestic sewage before being subjected to biological treatment. In recent days, solid state fermentation is also gaining importance to treat directly or with a limited amount of water supplementation to reduce the bioreactor volume, to avoid secondary clarifier, and to improve the process economics. In general, food-based, food industrybased, and agro-based wastes were extensively studied for energy generation (Elmekawy et al., 2014a). Although petroleum sludge treatment is also being evaluated for different bioprocesses, its energy generation potential is limited. Pretreatment of wastes is a process that converts organic matter to a biologically available form. Agro-based wastewaters contain high content of cellulosic materials and can be pretreated with biological (enzymatic solubilization), physical (grinding, ultrasound, thermal, and freeze-thawing), chemical (acid and alkaline), and physical-chemical (thermo-acid) processes (Guo et al., 2010; Nah, 2000; Marañón et al., 2012). Poultry and swine waste contains ammonia that inhibits the biological process, which requires the pretreatment for ammonia oxidation (Lei et al., 2007).

Methane

Bioplastics

Anoxic process

Anaerobic digesters

Mycoremediation

Wastewater/ waste remediation

Biodiesel Phycoremediation

Photofermentation

Microbial electrolysis

Hydrogen

Landfills

Microbial fuel cells

Solventogenesis

Dark fermentation

Hydrogen + VFA

FIGURE 2 Energy-generating processes in wastewater/waste remediation. [VFA, volatile fatty acid; ABE, Acetone–butanol–ethanol].

Bioelectricity

ABE

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Table 1 Various Biofuels Generated as the Value Added Products from Different Bioremediation Processes with Their Target Compounds/Pollutants/Substance Bioremediation Process

Treatment Output

Energy Output

Anaerobic digestion (AD) and landfills Acidogenic fermentation Photofermentation AD integration with acidogenic fermentation Heterotrophic microalgae cultivation

Organic matter removal from wastewater Organic matter removal from wastewater

Methane

Organic matter removal from wastewater Nutrients and organic matter removal from wastewater; CO2 reduction Waste/wastewater treatment Desalination Nutrient removal Waste/wastewater treatment Fermentation effluents Waste/wastewater treatment Carbon dioxide reduction Specific pollutant removal

Hythane


Microbial electrolysis cells Microbial electrosynthesis

Autotrophic metabolic functions Anoxic or microaerophilic process

Carbon dioxide and carbon monoxide, industrial emissions Wastewater Effluents of dark fermentation

Hydrogen

Lipids for biodiesel and carbohydrates for biofuels production Bioelectricity

Hydrogen Acetate Ethanol Butyrate Butanol Methane Biodiesel Bioplastics

2.1.2 Liquid waste The rate and volume of wastewater production is increasing continuously with increase in industrialization and human population. Based on the concentration and nature of pollutants, wastewater can be rated as high strength and low strength wastewaters as well as high and low biodegradable wastewaters. Wastewater from food-based industries, domestic origin, and agroindustries that contains a higher amount of biodegradable organic material can be treated efficiently (Zhang et al., 2014; Hamawand, 2015). The wastewater from chemical, pharmaceutical, metal- and rubber-based industries, which has a low fraction of biodegradable organic material, tends to have lower treatment efficiency. The carbon content of the wastewater also determines the potential of the wastewater toward application for energy generation. All types of wastewater are not suitable for all bioenergy-generating processes. The selection of bioenergy-generating process depends on the nature and composition of the wastewater. For example, distillery wastewater and dairy wastewater are suitable for methane generation by AD, biohydrogen production through dark fermentation, and bioelectricity generation through microbial fuel cells (MFCs), whereas domestic sewage is suitable for biohydrogen production by photofermentation and biodiesel production through microalgae cultivation (Mohan et al., 2011b; Zhou et al., 2012; Tyagi, 2013).


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2.2 BIOCATALYST 2.2.1 Whole cell Microorganisms have diverse metabolic functions that help in the degradation of a wide variety of substrates. Microorganisms ranging from eukaryotic algae and fungi to prokaryotic bacteria and archaea. These microorganisms are not only involved in the metabolic functions of degradation but also in the conversion of higher organic molecules to lower organic molecules and lower to higher organic molecules. Presentday technology has engineered such bioprocesses for various health, food, and environmental applications. Microbial environmental applications have been evaluated for wastewater/waste remediation; such a process is known as bioremediation or biodegradation. In the milieu of energy crisis and intense need for the world, various biodegradation and bioremediation technologies are being redirected toward simultaneous energy generation another advantage of biocatalyst is, reusability. Among all known bacterial processes for energy generation, the biocatalyst used for polyhydroxyalkanoates (PHAs) production cannot be reused as the product can be separated only through breaking the cell wall (Venkateswar Reddy et al., 2014; Jia et al., 2013). Similarly, in the case of lipid production from algae, the biocatalyst cannot be reused.

2.2.2 Mixed cultures Wastewater treatment process proceeds more comfortably with mixed bacterial cultures as the biocatalyst than any other pure bacteria strains (Pearce, 2003; Fang and Liu, 2002). The heterogeneity of the substrate and unsterilized conditions during the process are the main factors for the effective performance of mixed cultures (Mohanakrishna et al., 2010a). The importance of mixed cultures has become prominent as the waste remediation process is engineered to generate biofuels or/and chemicals and has led to the distinct field of mixed culture biotechnology (MCB). The diversity or composition of mixed culture is not similar for every waste-remediating process. Based on the process, the substrate and operating conditions, the microbial population corresponding to the specific process can be effectively enriched from the natural environment. Compared to pure culture-based energy or chemicals production, the advantages of MCB include: (1) no sterilization required, (2) adaptive capacity owing to microbial diversity, (3) generates a narrow product spectrum from the degradation of mixed substrate, (4) biologically robust system, and (5) continuous process with less complexity (Kleerebezem and van Loosdrecht, 2007; Mohan et al., 2007; Han et al., 2011).

2.3 BIOPROCESSES 2.3.1 Anaerobic process/AD AD is the classical bioprocess that works competently for removal of organic compounds from waste with simultaneous generation of a valuable energy product, methane. Even though the process was known for more than a century, it has been studied in depth only in the last 3 decades for different aspects of commercialization. AD is a process in which microorganisms oxidize the organic matter under anaerobic conditions into biogas (a mixture of methane (CH4) and CO2) (Angenent et al., 2004; Mohan et al., 2007). This basic process comprises four different phases in which the substrate undergoes biochemical changes: hydrolysis, fermentation/acidogenesis, acetogenesis, and methanogenesis (Figure 3). Since each phase of AD has individual importance with respect to metabolites and the AD process can be controlled at each phase for different forms of energy and chemicals production, it has become a versatile process in environmental biotechnology (Li and Yu, 2011). As shown in Figure 3, complex organic molecules such as polysaccharides, proteins, and lipids are hydrolyzed to their respective monomers (proteins to amino acids; polysaccharides to sugar; lipids to

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Organic matter (protiens, carbohydrates, lipids)

Hydrolysis Monomers and oligomers

Acetogenesis

enta Ferm

tion

H2 + CO2

Intermediates (propionate, butyrate, alcohols)

enta Ferm

tion

(amino acids, sugars, fatty acids)

Acetate

Methanogenesis Methane + CO2

FIGURE 3 A network of microbial processes of anaerobic digestion, treating complex wastewaters into CH4 through four different phases. Inhibiting the final step of methanogenesis, known as acidogenic fermentation, results in H2 and VFAs.

fatty acids) by hydrolyzing bacteria. This process is referred as hydrolysis. Through fermentation, fermentative bacteria convert the monomers to a mixture of low molecular weight organic acids (VFAs) and alcohols. Further, these fermentation products can be oxidized to acetic acid and hydrogen by hydrogen-producing acetogenic bacteria. This process, known as acetogenesis (Mohanakrishna et al., 2010b), also includes acetate production from the reduction of hydrogen and carbon dioxide. The bacterial group that reduces CO2 and H2 is known as homoacetogens. Finally, the acetate is converted to methane by methanogenesis; the bacterial group involved is acetoclastic methanogens. The pathways involved in methanogenesis constitute a complex metabolic network. On complete degradation of organic matter, methanogenesis ultimately leads to the production of CH4 and CO2. When the metabolism of methanogenesis is visualized in depth, this process utilizes three types of substrates. They are C1–C6 short chain fatty acids (also known as VFAs), n- or i-alcohols, and gases such as CO, CO2, and H2. Methane production can lead through two groups of methanogens. One group mainly uses acetate for methane production, whereas the second group uses H2 and CO2 or H2/CO2. During methane formation process, the coenzymes M play a significant role. If the formate and CO are the substrates for methane production, first CO and formate will be transformed to CO2 by coenzyme F420, and then CO2 will be reduced to methane by the action of coenzyme M. As this process alone has various pathways and the intermediary products have economic viability, understanding this metabolic network helps to engineer the process for the production of biohydrogen, VFAs, and alcohols. Integration with other biological processes for which the intermediates of AD act as substrates will lead to the production of alcohols, solvents, and PHAs, as visualized in Figure 3.


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2.3.2 Algal metabolism Algae are one of the oldest life forms on earth. Microalgae are unicellular and simple multicellular microorganisms that include both prokaryotic microalgae (cyanobacteria) and eukaryotic microalgae [green algae (Chlorophyta), red algae (Rhodophyta), and diatoms (Bacillariophyta)]. Eukaryotic algae have been found to be the most important classes of microalgae for biofuels generation. Algae can either be autotrophic (photosynthetic, requires only inorganic compounds such as CO2 and salts) or heterotrophic (nonphotosynthetic, requires organic source). Some photosynthetic algae are mixotrophic (they have ability to perform the processes) (Brennan and Owende, 2010). Several advantages of microalgae-derived biofuels can be foreseen, based on these attributes of microalgae: (1) capable of year round production; (2) require less water than terrestrial oil-producing crops, therefore reducing the load on freshwater sources; (3) can be cultivated in polluted or brackish water, therefore not compromising the production of food and arable land; (4) rapid growth rate (exponential growth rates is 3.5 h) and oil content will be in the range of 20-50% of dry cell weight; (5) higher potential for CO2 fixation (1.83 kg of CO2 can generate 1 kg of dry biomass); (6) wastewater acts as source for nutrients (nitrogen, phosphorus, and other micronutrients) required for microalgae cultivation; (7) microalgae can produce valuable coproducts like proteins and de-oiled biomass (after lipid extraction), which can be used as carbohydrate source for methane and ethanol production and as fertilizer. With these advantages, microalgae involves different types of biofuels generation such as biodiesel, biomethane, biohydrogen production with simultaneous accomplishment of wastewater treatment. Apart from these, cyanobacteria produce biohydrogen through photofermentation through treatment of effluents rich in organic acids. Most of the microalgae accumulate lipids under environmental stress conditions like nitrogen or phosphate limitation. Therefore such environmental conditions are controlled to improve microalgal lipid accumulation (Bellou et al., 2014; Mohan et al., 2011a; Amaro et al., 2011; Courchesne et al., 2009). The metabolic response of microalgal strains to the different wastewaters is not similar and has been found to depend on various factors such as type of energy source, pH, temperature, etc (Table 2). Figure 4 elucidates the detailed mechanism of lipid production in microalgae. Apart from lipid production, algae is also cultivated to capture the sunlight and atmospheric CO2 into biomass. Compared to simple structure of the biomolecules of the algae, it is being used as substrate for several bioenergy-generating processes such as biohydrogen, biomethane alcohols production, etc. (Turon et al., 2014). The residue of the algal biomass from biodiesel production process is rich in carbohydrate and was also found to have less protein content.

Table 2 Lipid Content for Biodiesel Production Through Microalgal Cultivation in Different Wastewaters Type of Wastewater or Waste

Lipid Content

Biocatalysts

References

Industrial wastewater Biogas plant effluent Cheese whey permeate Secondary treated sewage Dairy effluent

18.4% 35% 37.8 mg/L/day 17% 42%

Chlamydomonas sp. TAI-2 Monoraphidium sp. KMN5 Scenedesmus obliquus Botryococcus braunii Chlorococcum sp. RAP13

Anaerobic digestate of food wastewater

35.06%

Scenedesmus bijuga

Wu et al. (2012) Tale et al. (2014) Girard et al. (2014) Órpez et al. (2009) Ummalyma and Sukumaran (2014) Shin et al. (2014)

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Table 2 Lipid Content for Biodiesel Production Through Microalgal Cultivation in Different Wastewaters–Cont'd Type of Wastewater or Waste Synthetic feed (fructose, glucose, and acetate) Agro-industrial co-products Piggery wastewater Dairy and municipal wastewater Domestic wastewater Domestic wastewater Tertiary wastewater of CETP Primary treated wastewater

Lipid Content

Biocatalysts

References

52.6%

Scenedesmus sp.

Ren et al. (2013)

43% 6.3 mg/L/day 29%

Chlorella vulgaris Chlorella pyrenoidosa Mixed microalgae

Mitra et al. (2012) Wang et al. (2012) Woertz et al. (2009)

26% 28.2% 0.171 g lipid/g cell/day 0.132 g lipid/g cell/day

Mixed microalgae Mixed microalgae Chlorella minutissima Chlorella minutissima

Mohan et al. (2011a) Prathima Devi et al. (2012) Malla et al. (2015) Malla et al. (2015)

CETP, common effluent treatment plant.

CO2

Glucose

Sugars

Pyruvate

Mitochondrion Acetyl-CoA Citric acid Citrate cycle

Pyruvate

Citrate

Cytosol

Citrate ATP:CL

Calvin cycle

Structural lipids

Acetyl-CoA G3P

ACC

Malonyl-CoA

GPAT

FAT

Acyl-ACP

LACS

Acyl-CoA

PUFAs

G3P

Lysophosphatidic acid LPAAT

Membranes

FAS

Phosphatidic acid

Pyruvate

Structural lipids

KAS PDC

LPAT

Kennedy pathway

Diacylglycerol

Malonyl-ACP

DGAT ACC

Acetyl-CoA

Malonyl-CoA

TAG

Triacylglycerol

ER

Plastid

FIGURE 4 A simplified scheme showing lipid synthesis in microalgae (Bellou et al., 2014). ACC, acetyl-CoA carboxylase; ACP, acyl-carrier protein; LACS, long-chain acyl-CoA synthetase; ATP:CL, ATP-dependent citrate lyase; CoA, coenzyme A; DGAT, diacylglycerol acyltransferase; ER, endoplasmic reticulum; FAS, fatty acid synthase; FAT, fatty acyl-ACP thioesterase; G3P, glycerate-3-phosphate; GPAT, glycerol-3-phosphate acyltransferase; KAS, 3-ketoacyl-ACP synthase; LPAAT, lyso-phosphatidic acid acyltransferase; LPAT, lysophosphatidylcholine acyltransferase; PDC, pyruvate dehydrogenase complex; TAG, triacylglycerol.

3 BIOREMEDIATION PROCESSES THAT GENERATE ENERGY

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3 BIOREMEDIATION PROCESSES THAT GENERATE ENERGY 3.1 ANAEROBIC DIGESTION AD is a well-studied biological process that treats waste and wastewater to produce a methane-rich biogas. AD provides a spectrum of striking advantages such as pollution reduction and waste stabilization, energy recovery in the form of methane, cost-effective treatment process, suitable for a wide range of wastewaters and limited environmental impact. In addition, the solid remnants of the process can be used as fertilizer. With extensive research on various aspects of the AD process, it is now an established and commercially proven approach for treatment and recycling of biomass, waste, and wastewater (Mohan et al., 2008a; Kothari et al., 2014). Various categories of wastewaters and solid wastes from industries, agriculture, and domestic activities were used for the generation of methane through AD. Critical factors influencing the process were identified. In general terms, the CH4 production efficiency is dependent on the biodegradable nature of substrate. For example, the effluents of chemical, pharmaceutical wastewaters contain very low biodegradability and exhibit low efficiency for CH4 production. As the biodegradability increases, CH4 production potential increases. Biological/biochemical methane potential (BMP) is a standard test protocol that provides maximum CH4 of any substrate. The substrate needs to be evaluated for physical characteristics such as carbon oxygen demand (COD), biological oxygen demand (BOD), total solids (TS), volatile solids (VS), total carbon (TC), and total organic carbon (TOC) and then the BMP will be evaluated in batch tests. Operational parameters such as time, temperature, pH, etc., can be optimized from this BMP test (Mata-Alvarez et al., 2014). During the operation of anaerobic reactor, the different categories of complex waste/substrate (carbohydrate, lipids, and protein) should simultaneously digest all organic substrates in a single bioreactor. The composition of all the wastes/wastewaters is not identical. In general to AD, the key parameters that govern the process are temperature, VFAs, pH, ammonia, nutrients, trace elements. Additional parameters might be included, based on the composition and nature of the waste (Appels et al., 2011). Temperature is the most significant factor that influences microbial activity, which in turn influences the CH4 and quality of the effluent/digestate. Among the three thermo-dependent bacterial groups, mesophilic bacteria that perform at 30-40°C condition are observed in many digesters. However, methane yield improves with increase in temperature. Thermophilic bacteria have accelerated metabolic rates and higher growth rates, which improves the biogas formation (El-Mashad et al., 2004; Sánchez et al., 2001). Biogas production under thermophilic condition (55 °C) is found to be double that of psychrophilic condition (15 °C), thus highlighting the key role of temperature (Sánchez et al., 2001). Higher temperature also increases degradation of substrates by benefiting endergonic reactions. Organic nitrogen degradation and phosphorus assimilation were also found to increase with increase in temperature. VFAs are intermediary products of AD, which include acetic acid, propionic acid, butyric acid, and valeric acid. By the action of syntrophic acetogenic bacteria, these VFAs can be converted to CH4. Among the four VFAs, propionic acid to acetic acid ratio is considered as the indicator for the performance of the AD. The higher value of this ratio infers digestion imbalance (Marchaim and Krause, 1993; Buyukkamaci and Filibeli, 2004). VFA determines the system pH, and it is observed that both VFA concentration and pH are dependent factors. pH in the range of 6.5-7.2 was observed to be optimum for methanogenesis. Higher VFA concentration leads to decrease in pH and acts as vice versa phenomenon. The rate of shift in pH with respect to VFA concentration change is also dependent on the

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buffering nature of the bulk liquid. High buffering systems provide more stability with change in VFA concentration. C/N ratio is another key parameter needed for the optimum nutrient balance required for the growth of bacteria. Generally, the C/N ratio in the range of 20-30, is considered as optimum (Li et al., 2011; Puyuelo et al., 2011). If the C/N ratio of any waste is not suitable for methanogenesis, a co-substrate addition strategy can maintain optimum ratio in the digester. Waste or wastewater from animal origin such as meat-processing wastewater or manure of swine or poultry contains high protein content. With treatment in anaerobic digester, it generates ammonia. The ammonia generation process consumes essential nutrients required for bacterial growth. Also, ammonia in higher concentration is toxic to the microbial metabolism.

3.2 DARK FERMENTATION 3.2.1 Biohydrogen Biohydrogen production through dark fermentation is referred as the truncated version of methanogenesis. During methanogenesis, if the final/fourth stage is inhibited or stopped, the end products of the acetogenesis (one of the intermediary steps of AD) are H2 and acetate. Based on the microbiological and operational behavior of the biohydrogen production process, several strategies were developed. Compared to pure cultures, mixed cultures perform better for hydrogen production from wastewater or waste remediation. This finding led to the enrichment of hydrogen-producing bacterial consortia from the methane-producing (anaerobic) consortia. Treating anaerobic sludge with acid, alkaline, heat, and chemical (BESA, bromoethane sulphonic acid) were proved as efficient methods (Wang and Wan, 2009; Mohan et al., 2007, 2008c; 2012a,b; Mohanakrishna et al., 2011). The common mechanism behind these methods is elimination of methanogens from the total bacterial consortia of AD for methane production. The methanogens are spore-forming bacteria; they are sensitive to extreme environments such as heat, acid, and alkaline conditions, whereas BESA inhibits enzyme cofactor, which plays a key role in methanogenesis (Mohanakrishna and Mohan, 2013; Wang and Wan, 2009). Optimum operational conditions of the process such as pH, time of operation, VFA concentration, and organic loading rate were used to control the hydrogen-producing system for maximum production efficiencies. Mild acidic conditions (pH 5.5-6.5), lower retention time (than methanogenesis), and mild VFA concentrations are optimum operation conditions. Compared to a continuous mode, batch mode operation is very much suitable for hydrogen production. Several categories of industrial wastewaters, viz., food processing, dairy-based, alcohol-based, plant and agricultural-based wastewater, were evaluated and optimized for the hydrogen production. However, H2 production and treatment efficiencies vary with each type of waste. Similar to AD, biohydrogen production process was also influenced by various factors like organic loading rate, temperature, pH, etc., (Zhu and Beland, 2006). Compared to AD, dark fermentation is found to have less substrate degradation efficiency. The final product, hydrogen, generates along with the VFAs and will not be further degraded to carbon dioxide. Since the carbon persist in the VFAs, the effluent of the hydrogen process contains at least 70% of the carbon residue. Practically, during bioreactor operation, various other microbial metabolisms also act on the organic matter, and because of it the maximum residual organic matter is found around 30-35% (maximum substrate degradation efficiency, 65-70%). Higher concentrations of residual VFA in dark fermentation against the targeted research focus of complete waste degradation, has directed towards the bioprocess integration approach, which, will be discussed in later sections.


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3.3 PHOTOFERMENTATION Apart from the dark fermentation, photofermentation is one of the possible alternatives for energy generation from waste streams. Photofermentation is the process of fermentative conversion of organic carbon, in the presence of light unlike dark fermentation, manifested mostly by photosynthetic bacteria (PSB) similar to anaerobic fermentation. Photofermentation is most studied for H2 production with respect to bioenergy generation, especially as an integration to the acidogenic fermentation to valorize the residual organic fraction rich in volatile acid intermediates (Srikanth et al., 2009a; Srikanth et al., 2009b; Mohan et al., 2013a). Processing of unutilized carbon sources in wastewater for additional H2 production will sustain practical applicability of the process. Combination of dark and photofermentation processes is considered to be an ideal route leading to nearly the highest theoretically possible yield (Srikanth et al., 2009a; Srikanth et al., 2009b). Photobiological H2 production is feasible through two diverse photosynthetic mechanisms, oxygenic and anoxygenic. Oxygenic photosynthesis is catalyzed by microalgae and cyanobacteria, while anoxygenic photosynthesis is catalyzed by PSB. Anoxygenic process for H2 production is advantageous over oxygenic process due to the absence of oxygen, which is considered to be a scavenging molecule. During oxygenic photosynthesis, O2 gets released as a by-product during photolysis of water, where H2 gets generated through either direct or indirect photolysis by hydrogenase, with a marginal variation in the mechanism (Kruse et al., 2005; Beer et al., 2009). Generally, hydrogenase function is based on the O2 availability in the system and supply of e− and H+, either directly by photosynthetic water splitting (driven by photosystem (PSII)) or indirectly from the degradation of organic molecules (starch and nitrogen fixation). Hydrogenase acts as H+/e− release valve by recombining H+ (from the medium) and e− (from reduced ferredoxin) to produce H2. Nitrogenase can also catalyze the H2 production during nitrogen fixation but the process is extremely O2 sensitive. Localization of nitrogenase in the heterocysts of filamentous cyanobacteria protects it from O2 (Vyas, 1995). Further investigations on this process also revealed that the H2 production in cyanobacteria is stimulated by nitrogen starvation and is catalyzed by “reversible hydrogenase” or “bidirectional hydrogenases” (Horch et al., 2012). Bidirectional hydrogenases are mainly involved in the disposal of excess reducing power derived from fermentation and photosynthesis resulting in H2 evolution. PSB are potent H2 producers compared to cyanobacteria in the presence of sunlight as they neither use water as an e− source nor produce O2 photosynthetically. PSB can use a variety of carbon sources (carbohydrates and VFA), and the process is strictly anoxygenic in nature (Wilhelm, 1996; Mohan et al., 2013a). The enzymes responsible for H2 production in PSB are also hydrogenase and nitrogenase, but their activities are not inhibited due to the nonproduction of O2. The efficiency of light energy conversion to H2 by PSB is much higher than cyanobacteria due to less quantum of light energy requirement than the water photolysis (Batyrova et al., 2012; Vyas, 1995). Among the photosynthetic processes of H2 production, the photofermentation route seems to be favorable due to relatively higher substrate to H2 yields, its ability to trap energy over a wide range of the light spectrum, and versatility in sources of metabolic substrates (Srikanth et al., 2009a).

3.4 BIOELECTROCHEMICAL SYSTEMS Research on bioenergy generation from renewable feedstocks has discovered the most transdisciplinary system with high treatment efficiencies along with multifaceted applications; namely, bioelectrochemical systems (BES) (Pant et al., 2012). BES works at the interface of fermentation and electrochemistry as basic

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research areas, although various other sciences also contribute to make the system complete. The applications of BES include current generation, bioremediation of a wide range of wastes, specific pollutants/ xenobiotics removal, desalination, color removal, toxicity reduction, gaseous pollutants treatment, synthesis of commercially viable chemicals and solvents, CO2 reduction, etc. (Venkata Mohan et al., 2014a). In this chapter, however, the focus is more on considering waste as bioenergy source based on three wellestablished forms of BES: MFCs, microbial electrolysis cell (MEC), and microbial desalination cell (MDC).

3.4.1 Microbial fuel cells Harnessing of bioelectricity from waste streams using MFC has garnered significant interest in both basic and applied research, in the recent past, due to its sustainable and renewable nature. Creating an electrical circuit between e− source (substrates) and the e− sink (molecular oxygen) by placing noncatalyzed electrodes facilitates the development of a potential gradient against which the e− flow in the circuit and can be harnessed as bioelectricity (Venkata Mohan et al., 2014a; Pant et al., 2012; ElMekawy et al., 2013b). MFC facilitates direct transformation of chemical energy stored in the waste to electrical energy via microbial catalyzed redox reactions under ambient temperature and pressure (Figure 5). Anodic oxidation and cathodic reduction reactions are the basis for MFC function. The reducing equivalents H+ and e− generated through a series of bioelectrochemical redox reactions at anode (Eq. 1) will reach the cathode and become reduced in the presence of an electron acceptor (Eq. 2).

Effluent

C6 H12 O6 + 6H 2 O → 6CO2 + 24H + + 24e − ( Anode )

(1)

4e − + 4H + + O2 → 2H 2 O (Cathode )

(2)

C6 H12 O6 + 6H 2 O + 6O2 → 6CO2 + 12H 2 O (Overall )

(3)

Anodic oxidation

CO2

Cathodic reduction

e

–

e

Wastewater

TEA (O2) CO2/effluent pollutant

H Wastewater

–

Reduced TEA (H2O) Product synthesis Treated pollutant/waste

+

Biotic/abiotic

Biotic Membrane

FIGURE 5 Schematic of a typical Bioelectrochemical system with multiple possible application. [TEA, terminal electron acceptor].


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In general, the anode chamber of MFC is operated under anaerobic condition, but few reports are available in the literature pertaining to the application of aerobic or microaerophilic metabolic function at anode (Ringeisen et al., 2007; Rodrigo et al., 2010; Mohan et al., 2008b). Apart from power generation, MFC application has been extended toward multiple functions including waste remediation, specific pollutants removal, and recovery of value-added products based on the electron donor and acceptor conditions, bringing these functions together under one heading, BES. In fact, MFC operation was mostly reported with respect to waste remediation and removal of specific pollutants, rather than power generation. Combined function of all the biological, physical, and chemical components of MFC provides an opportunity to trigger multiple reactions at once as a result of microbial metabolism and subsequent secondary reactions (Mohan and Srikanth, 2011). The anode chamber of MFC mimics the functions of both conventional anaerobic bioreactor and electrochemical unit, where the combination of these redox reactions helps in enhancing the degradation of organic matter and toxic or xenobiotic pollutants. Similarly, the cathode chamber also depicts effective waste treatment with in situ generation of strong oxidants. The potential difference between anodic oxidation and cathodic reduction reactions helps in the enhancement of the degradation of different pollutants at both the electrodes. Synergistic interaction between the MFC components and the biocatalyst needs to be understood and optimized to fully exploit the capacities of these systems in order to maximize the treatment efficiency and energy generation (Srikanth et al., 2011). Effective utilization of waste and specific pollutants as substrates in MFC has the dual advantage of bioenergy generation as well as waste treatment. Origin and composition of waste can influence the treatment efficiency of MFC as well as energy recovery. A diverse range of wastewaters has been studied as substrates in MFC from domestic to industrial origin (Pant et al., 2010; Mohan et al., 2013b). Table 3 presents a detailed list of waste used in MFC as substrate and the degree of treatment efficiencies reported. However, the efficiency of electron recovery depends on the oxidation state of electron donor and the ratio of the electron donor to the microbe that can oxidize it. Waste having higher biodegradability, from dairy, food, vegetable, kitchen, etc., origin, will have higher power generation capacity, over the waste originated from complex industrial environments with low biodegradability Table 3 Various Food Industry Based Wastewater Used as Anodic Fuels in MFC and Their Respective Performances Wastewater

Power Density

ξCOD (%)

References

Chemical wastewater

339.87-862.85 mA/m2

55.76-62.9

Slaughter house wastewater Animal carcass wastewater Swine wastewater Beer brewery wastewater Beer brewery wastewater Dairy wastewater Dairy wastewater

578 mW/m2 2.19 W/m3 228 mW/m2 483 mW/m2 264 mW/m2 1.1 W/m3 (~36 mW/m2) 161 mW/m2

93 ± 1 50.66 84 87 43 95.49 90

Cheese whey Chocolate industry wastewater

1.3 ± 0.5 W/m3 1500 mW/m2

59.0 ± 9.3 74.77

Mohan et al. (2008c,d) and Raghavulu et al. (2009) Katuri et al. (2012) Li et al. (2013b) Kim et al. (2008) Wang et al. (2008) Wen et al. (2009) Venkata Mohan et al. (2010a) Elakkiya and Matheswaran (2013) Kelly and He (2014) Patil et al. (2009)

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Table 3 Various Food Industry-Based Wastewater Used as Anodic Fuels in MFC and Their Respective Performances–Cont'd Wastewater

Power Density

ξCOD (%)

References

Molasses wastewater Distillery wastewater

1410 mW/m 124.35-245.34 mW/m2

53.2 49.32-72.84

Fermented distillery wastewater Molasses wastewater mixed with sewage Palm oil mill effluent Vegetable waste Fermented vegetable waste Cereal-processing wastewater Canteen based food waste Food waste leachate Mustard tuber wastewater Protein food industry wastewater Cattle dung Cattle manure slurry Cow manure Diluted wheat straw hydrolysate Steam exploded corn stover hydrolysate Powdered raw corn stover Steam exploded corn stover residue Rice straw hydrolysate

224.93 mA/m2

86.67

Zhang et al. (2009) Mohanakrishna et al. (2010a, 2012) Mohan et al. (2011c)

382 mW/m2

59

Sevda et al. (2013)

44.6 mW/m2 57.38 mW/m2 111.76 mW/m2 81 ± 7 mW/m2 ~556 mW/m2 432 mW/m3 246 mW/m2 45 mW/m2

~90 62.86 80 95 86.4 87-92 57.1 86

Cheng et al. (2010) Venkata Mohan et al. (2010b) Mohanakrishna et al. (2010b) Oh and Logan (2005) Jia et al. (2013b) Li et al. (2013c) Guo et al. (2013) Mansoorian et al. (2013)

220 mW/m3 765 mW/m2 349 ± 39 mW/m2 148 mW/m2

73.9 ± 1.8 41.9-56.7 ~50 (carbon) 95 (xylan and glucan)

Zhao et al. (2012) Inoue et al. (2013) Wang et al. (2014a) Thygesen et al. (2011)

367 ± 13 mW/m2

94 ± 1

Zuo et al. (2006)

331 mW/m2 406 mW/m2

42 ± 8 (cellulose) 60 ± 4 (cellulose)

Wang et al. (2009) Wang et al. (2009)

137.6 ± 15.5 mW/m2

49.4-72

Wang et al. (2014b)

2

(ElMekawy et al., 2014a). Still, the wastewater could be a potential carbon source for MFC due to the possibility of converting negative-valued waste into bioenergy. However, optimization is still required for upscaling the process with concerted efforts (Pasupuleti et al., 2015). MFC was also reported as a secondary integration unit to the fermentation and preliminary treatment processes, to treat the residual organics present in the effluent. Few studies have been reported in the literature based on this concept but only a very few of them used real field effluents, and the coulombic efficiencies are between 12% and 45%. The biocatalyst enriched in presence of acid metabolites was reported to depict higher treatment efficiencies and power output (ElMekawy et al., 2014b) due to the adaptation of biocatalyst. Treatment gained in this type of integrated system is an addition to the first process, increasing the total valorization capacity of the waste. Apart from wastewater and effluents, water with specific pollutants can also contribute to energy generation in MFC with their electron donor and acceptor functions. In general, the chemotrophic (autotrophic/heterotrophic) microbes can utilize various inorganic components as electron donors for


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their metabolism, where some of the pollutants of wastewater can take the donor job and oxidize them at anode. Similarly, a few biocatalysts can also utilize these pollutants as electron acceptors in respiration (in absence of O2) at cathode, facilitating their remediation. Apart from this, some pollutants such as sulfur, metals, estrogens, etc., can act as redox mediators for the electron transfer at anode (Kumar et al., 2012). Removal of sulfides (Rabaey et al., 2006), nitrates (Clauwaert et al., 2007; Virdis et al., 2008), perchlorate (Butler et al., 2010), chlorinated organic compounds (Venkata Mohan et al., 2014b), and estrogens (Kumar et al., 2012) was reported in MFC along with power generation. Some of the known metal pollutants such as iron, manganese, selenium, uranium, chromium, arsenic, vanadium, and cobalt can also enhance the power generation by acting as electron acceptors. Also, the colored dye compounds can act as alternate electron acceptors resulting in power generation and their removal. Nitrobenzenes, polyalcohols, and phenols were also studied for their treatment in MFC. Similar to fermentation, MFC can also be operated using both dark and photosynthetic modes. However, very few and specific reports are available on the usage of photosynthetic mechanism in MFC operation (ElMekawy et al., 2014c). Most of these studies are related to single strains of green algae, cyanobacteria, and PSB acting as anodic biocatalyst under photoautotrophic or photoheterotrophic mode. Energy from the sun will be used to activate the photosystem and an organic or inorganic electron source will be used to generate energy. Light energy captured by pigment molecules channels to the bacteriochlorophyll (Bchl) and triggers a series of photochemical reactions separating the positive and negative charges across the membrane (Chandra et al., 2012). This charge separation initiates electron transfer coupled to the translocation of protons across the membrane, generating a potential gradient and resulting in electrogenesis (Mohan et al., 2013a). In algae, the mechanism of electrogenesis is little different and is based on membrane-bound protein complexes (photosystem (PSI), PSII, and cytochrome bf complex) and O2 generating (oxygenic photosynthesis). Henceforth, the power generation will be on the lower side but the treatment efficiency is on the higher side compared to the anoxygenic photosynthesis (bacterial). Chlamydomonas reinhardtii, Phormidium, Nostoc, Spirulina, Anabaena, Synechocystis PCC-6803, etc., are the most studied species with oxygenic photosynthesis in MFC (Mohan et al., 2013a; Rosenbaum et al., 2010), while anoxygenic photosynthesis was evaluated using PSB like Rhodopseudomonas palustris, Rhodobacter sphaeroides, Rhodobacter, Rhodopseudomonas, etc., as biocatalyst in MFC (Rosenbaum et al., 2010; Mohan et al., 2013a). The application of oxygenic photosynthesis was also studied at cathode of MFC (algal biocathode), where the in situ-generated oxygen replaces the mechanical aeration that reduces the overall input energy for the system (Mohan et al., 2009; Venkata Mohan et al., 2014c).

3.4.2 Microbial electrolysis cells Hydrogen is one of the alternative fuels having high impact in the present bioenergy research due to its high energy value (122 KJ/kg) and non-emission of greenhouse gases at combustion. The biological route of H2 production involves more sustainable chemical, physical, and electrochemical methods due to less energy input needed and the possibility of avoiding pollution during its production (Mohan et al., 2011b). Furthermore, the carbon source present in wastewater can also be considered as substrate for the biological H2 production due to its dual benefits, H2 production and waste remediation. An enormous amount of literature is available on the biological route of H2 production, and the technology is well established and understood with respect to the operational and regulating factors (Mohan et al., 2011b). In the course of understanding the process, various constraints such as dominance of methanogenic activity, low substrate conversion efficiency, accumulation of acid metabolites, sudden pH drop, etc., were observed (Srikanth and Venkata Mohan, 2014). Different strategies were also developed to overcome these constraints such

552


as selective enrichment of inoculum (Srikanth et al., 2010), integration of secondary systems such as photofermentation (Srikanth et al., 2009a), MFC (ElMekawy et al., 2014b; Mohanakrishna et al., 2010b), methanogenesis (Mohanakrishna and Mohan, 2013), PHAs production (Venkateswar Reddy and Venkata Mohan, 2012), etc. In summary, the fermentative H2 production alone cannot compete with the existing wastewater treatment methodologies because of the limited treatment ability. In this context, researchers tried to combine the acidogenic fermentation with the electrolysis cell, to reduce the energy required for the water hydrolysis through microbial metabolism, naming this process MECs (Jeremiasse et al., 2011; Jeremiasse, 2011; Logan et al., 2008; Cheng and Logan, 2007). MEC is based on the principle that the reducing equivalents generated at anode from the oxidation of organic matter will get reduced to hydrogen at cathode (Liu et al., 2005). However, the supply of a small amount of external energy makes the process cross the endothermic barrier to form H2. The standard redox potential for H2 formation is −0.414 V, which is not possible to generate from microbial metabolism, and hence an additional energy supply is required to favor the formation of H2. Low energy consumption compared to water electrolysis and high product (H2) formation and substrate degradation rather than fermentative process make MEC a potential replacement for the existing methods and escalating energy demands. The performance of MEC has significantly improved within just a few years of its discovery (Logan et al., 2008; Liu et al., 2005). A few research groups also started working toward integrating the wastewater treatment with MEC to define an economically feasible hydrogen production methodology. A few pilot-scale operations also have been demonstrated, in spite of some drawbacks in process optimization (Cusick et al., 2011). MEC was also reported to utilize the acid-rich effluents generated from fermentation bioreactor (Lalaurette et al., 2009). Further to this research, a new process has been developed to convert the inherent energy of the wastewater into commercially viable commodities. Using the protons and electrons (energy) generated in the system, the microbial reduction process is being carried out in similar kinds of systems instead of generating H2, naming the process microbial electrosynthesis (Pant et al., 2012; Venkata Mohan et al., 2014a; Rabaey and Rozendal, 2010; Sharma et al., 2013). CO2 from the atmosphere and industrial off-gases is being considered as a major carbon source for this process. However, substantial input is needed to understand this process and make it a potential tool for mitigating environmental pollution and complementing global energy demands.

3.4.3 Microbial desalination cells MDCs are one of the forms of BES and are being considered a promising technology for clean water and energy production (ElMekawy et al., 2014d). MDC technology is derived from MFC, where a third chamber is introduced between anode and cathode chambers to drive the energy towards water desalination. The ion exchange membranes, anion exchange membrane (AEM) and cation exchange membrane (CEM), used on both sides of middle chamber (filled with saline water) separating the anode and cathode, respectively, will do the job of desalination by separating the ions based on charge toward respective electrodes (anions to anode and cations to cathode) as depicted in Figure 6. Anodic biofilm triggers the electron flow in the circuit by oxidizing the organics and deploys the migration of anions and cations from the middle chamber with simultaneous power generation (Jacobson et al., 2011; Cao et al., 2009; Mehanna et al., 2010). Migration of ions in the MDC is powered by the potential difference between the electrodes, and this process can reach a desalination level up to 99% along with energy generation (Jacobson et al., 2011; Cao et al., 2009). The first report on MDC was published by Cao and his coworkers in 2009 with a volume of 3 mL, which later increased to 1 L by Jacobson and his coworkers (9-11).


Anodic oxidation

Effluent

CO2

Ca+

An–

Ca+

e–

Cathodic reduction

e–

Substrate

Wastewater

An–

An–

Ca+

An–

Ca+

An–

Ca+

Biotic

553

Reduced TEA

TEA

Biotic/abiotic AEM

CEM

FIGURE 6 Microbial desalination cell (MDC) depicting removal of salts through three-chambered design.

Since then, the MDC technology has evolved on several fronts including reactor design, process optimization, and productivity. Most of the MDC studies are based on synthetic/designed salt waters, and a very few studies were based on brackish water or real seawater. However, this technology is proven to treat high concentration of salt water (35 g/L) with a desalination efficiency of 93%. Experimental output from several studies confirmed the high desalination efficiency of MDC and its suitability for being utilized as either a stand-alone technology or pretreatment process for the conventional desalination technologies. Various attempts were also made to improve the charge transfer, balance the pH in anode and cathode, increase the desalination efficiency and continuous operation of the cell (ElMekawy et al., 2014d). Irrespective of its advantages, MDC technology has various limitations, including the inability to concentrate huge amounts of acid or salt solutions by ion exchange process, prolonged time needed due to the decelerated anodic metabolism (Brastad and He, 2013), competitive ions other than those representing hardness, gradual increment of salt concentration in the anode and cathode compartments, etc. Overall, the MDC technology can be more effective in desalinating the water along with power generation, with optimal operating conditions.

3.5 ANOXIC PROCESS AND BIOPLASTICS In wastewater treatment, the bacteria is subjected to feast and famine regimes. It is very often observed during sequencing batch processing of wastewater. When the substrate is present in excess or under stress conditions, the bacteria accumulates the storage polymers in the cytoplasm (Amulya et al., 2014; Bengtsson et al., 2008). Known as PHAs, these are considered as bioplastics from bacterial origin. Under external substrate limited conditions, these storage polymers depleted within the bacterial cell provide required energy for the cell metabolism. The accumulation of biopolymers further engineered for the large-scale production of bioplastics. PHAs are the polymers of hydroxy fatty acids that are

554


naturally produced by specific groups of bacteria (Moralejo-Gárate et al., 2014). PHAs are group of polymers in which the polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV) were found as dominant and having suitable physicochemical properties to use as bioplastics. In general the PHA is produced under anoxic or microaerophilic conditions (low dissolved oxygen in bulk liquid) and in association with a few other factors: high substrate availability, nutrient limitation, and high biomass concentration. Three different pathways were reported for the PHA synthesis. In the primary pathway the acetyl-CoA acts as the precursor on which the PHA synthase plays a crucial role for the PHA production. Here, acetyl-CoA is converted into 3-hydroxybutyrylCoA and subsequently polymerized to PHB by the PHA synthase. In another pathway, PHA is produced as an intermediate of the β-oxidation process in which the alkanes, alkanols, and alkanoates act as the substrates. The type of PHA polymer depends on the type of substrate. Fatty acid de novo biosynthesis pathway is the third pathway that produces PHA, in which various types of hydroxyalkanoates are produced from simple carbon sources like acetate, fructose, glycerol, and lactate (Urtuvia et al., 2014). PHA production was well studied in wastewater treatment plants that are processing a wide variety of wastewaters (Table 4). Established industrial processes consume expensive sugars like glucose, sucrose, or other agri-products such as corn for PHA production (Venkateswar Reddy and Venkata, 2012). Utilizing inexpensive substrates such as waste, wastewater, and microbial consortia as catalyst for PHA production is making the process economical (Lemos et al., 2006). In recent years, this is one of the reasons for decreasing the polymer production cost. In general, during the bacterial energy generation processes through fermentation, the product is exocellular, and the biocatalyst can be reused or regenerated. In this case, the PHA production happens at the expense of biomass (biocatalyst). In terms

Table 4

Bioplastics Production from Wastewater and Waste (PHB or Any Other)

Type of Wastewater or Waste

PHA Production Efficiency

Candy bar factory

0.70 gPHA/gVSS

Municipal wastewater Dairy industry wastewater Food industry wastewater Excess sludge fermentation liquid Wood mill effluents Spent wash effluents Acidogenic fermented food waste Unfermented food waste Feedstock from pulp industry DCW, dry cell weight.

Biocatalysts

References Tamis et al. (2014)

25% of dry biomass

Plasticicumulans acidivorans Mixed consortia

0.5 gPHA/gVSS 0.60 gPHA/gVSS 59.5% of DCW

Mixed consortia Mixed consortia Mixed consortia

37% of storage yield 40% of DCW 59% of DCW

Mixed consortia Bacillus tequilensis Bacillus tequilensis

35.6% of DCW 67.6% of DCW

Aerobic mixed culture Mixed microbial culture

Morgan-Sagastume et al. (2010) Chakravarty et al. (2010) Anterrieu et al. (2014) Jia et al. (2014) Mato et al. (2010) Amulya et al. (2014) Venkateswar Reddy et al. (2014) Reddy and Mohan (2012) Queirós et al. (2014)


555

of energy, the regeneration of new biomass is consuming more energy. Composition of the PHA polymer is determined by the different metabolisms that prevail in the bacteria/microbial consortia. PHB, PHV, and copolymer of PHB-PHV are dominant polymers in PHA group. PHV is the main product from propionate, whereas PHB is the only product from acetate. Yield of PHB-PHV mixtures and its ratio is directly related to the acetate and propionate uptake rate (Jiang et al., 2011). Pilot-scale operations have also been conducted for the synthesis of PHA from different types of wastewaters. A successful demonstration at a Mars candy factory in the Netherlands using Plasticicumulans acidivorans as biocatalyst reported higher PHA production of 0.70 gPHA/gVSS (volatile suspended solids) within 4 h (Tamis et al., 2014; Figure 7). PHA was produced from the different types of wastewater that are originated from the food and agro-product-based industries, which have different types of carbon sources ranging from acetate to glucose and sucrose. Compared to hexoses and pentoses, simple substrates such as VFAs are more suitable for the PHA production. In this direction the effluents of biohydrogen production process are used as substrate. It resulted in at least 10% higher accumulation of PHA in bacterial cell (Reddy and Mohan, 2012).

Chocolate

VFA

Mars Factory Waste Fat removal

Fat separation unit

First anaerobic (USB) reactor

Second anaerobic reactor

Wastewater To WWTP

Fat removal

VFA

Pilot influent tank

Enrichment reactor

Microbial enrichment culture

Accumulation reactor

Effluent

Biomass with high PHA content

FIGURE 7 Overview of the pilot-scale system for PHA production from wastewater at the Mars candy bar factory, Veghel, The Netherlands (Tamis et al., 2014). [WWTP, wastewater treatment plant; USB, upflow sludge blanket].

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4 ENERGY GENERATION THROUGH BIOPROCESS INTEGRATIONS The yield exhibited by any process is always different from the theoretical yield due to several operational, biological, and environmental factors. Complete oxidation of the waste or residual organics by secondary processes yields additional value-added products (Guwy et al., 2011; Laurinavichene et al., 2010; Mohanakrishna and Mohan, 2013). Sequential integration of one bioenergy-generating process to another bioenergy-generating process has led to the biorefinery approach, which improves the degradation efficiency, process economics. The integration of one process to another is selected based on the substrate nature and the possible high energy yield in individual process. The best examples of such an integration approaches were shown in Figure 8, where dark fermentation is integrated with five other processes such as solventogenesis (for alcohols), MFC (for bioelectricity), MEC (for hydrogen), photofermentation (for hydrogen), and anoxic process (for bioplastics). The maximum reported substrate degradation using dark fermentation alone is 70%. An effluent of effective dark fermentation is rich in VFAs and the integrated five processes were proved to work efficiently with VFA-rich substrates. The residual pollutants of the primary effluent were further treated in secondary process, and the secondary effluents were found to have lesser organic matter. Based on the residual organic matter and nature of composition, tertiary treatment is also possible for the additional energy generation (Mohanakrishna and Mohan, 2013). Dark fermentation, photofermentation, and methanogenesis were integrated in 11 different combinations and evaluated for substrate degradation efficiency and energy generation from each combination. Results showed 87.5% of substrate degradation and 85.3% of carbon conversion efficiency toward H2 and CH4 (Mohanakrishna and Mohan, 2013). Methane

Anaerobic digestion Wastewater/ waste remediation

Solventogenesis

Acidogenic fermentation

H2


Alcohols

Bioelectricity

Microbial electrolysis cell Hydrogen Photofermentation

VFA Anoxic process

Bioplastics

FIGURE 8 Dark fermentation-centered bioprocess integration diagram explaining various other energy-generating processes that can be integrated with biohydrogen production process.

REFERENCES

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5 FUTURE PERSPECTIVES Removing gaseous pollutants through biological processes is less studied. CO2 fixation for energy generation is going to be a pioneer technology in the near future. Microalgae processes for biomass and biodiesel production is one of the major available routes. In addition, the very recent concept of microbial electrosynthesis (MES) has foreseen several advantages such as CO2 fixation with simultaneous bio-commodities synthesis with low energy demand. This process can be integrated with the other renewable energy-generating technologies. Biorefinery concepts that integrate different bioprocesses for different energy are at infant stage. A comprehensive program for the technoeconomic evaluation for such processes needs to be developed for commercial scale. Metabolic engineering for biorefineries needs to be simulated at the laboratory scale to achieve broad and rapid understanding on energy efficiencies. On the other hand, molecular, genetic tools accelerate designed microorganisms for remediation and simultaneous energy generation. The knowledge of the microbiological pathways and enzymes that involves energy generation from waste and their metabolic pathways can be advantageous in the quest for alternative energy (Singh et al., 2008).

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